1. Field of the Invention
This invention relates to miscible blends of polymeric compositions. Particularly, this invention relates to miscible blends of thermoplastic and elastomeric compositions having lower critical solution temperature (LCST) behavior and to methods of utilizing this phenomenon to enhance the properties of such compositions. More particularly, the present invention relates to miscible blends of chlorinated polyvinyl chloride and copolymers of acrylonitrile-butadiene such that blends of the two manifest LCST behavior in a practically achievable temperature range.
2. Background of the Art
Chlorinated polyvinyl chloride (CPVC) has become an important specialty polymer due to its high glass transition temperature, high heat distortion temperature, chemical inertness and outstanding flame resistance. Typically, commercial CPVC has in excess of about 57 percent (by weight) chlorine and can be prepared by well-known techniques, such as by the suspension chlorination of polyvinyl chloride, for example. The CPVC resins so-made can be processed and formed by conventional methods such as by milling, calendering, extruding, laminating, molding and the like. However, a well-known undesirable characteristic of CPVC is that as the chlorine content of the polymer is increased, the resin becomes more difficult to process. The increased chlorine content results in a hardening or stiffening of the polymer so that its melt flow is greatly reduced at a given temperature. Consequently, the polymer is difficult to process, mold and shape without the addition of modifying additives. Moreover, another well-known undesirable property of CPVC is that it has inherently low impact properties.
To overcome these problems it has been proposed to add to the CPVC modifying additives such as processing aids and impact modifiers. A processing aid is used in melt blending CPVC resin to hasten fusion, to smooth out the otherwise rough texture of the resin, and to soften the resin so that it is uniformly melt blended within an acceptably short period of time. Also, since the end use product must have desirable impact strength, impact modifiers are also essential to a CPVC composition. These efforts, however, have ordinarily proven unsatisfactory because any improvement achieved in one property has frequently been accomplished through the undue sacrifice of other desirable properties.
For example, impact modifiers are incompatible or semi-compatible discontinuous domains of rubbery-type elastomers that are homogeneously dispersed throughout a continuous thermoplastic phase. For optimum performance, the impact modifier must resist the formation of a single phase system within the thermoplastic which it is dispersed. The utilization of the elastomeric impact modifiers contribute little, if any, improvement in processibility, and sometimes even increase the melt elasticity of the CPVC composition. Generally, most impact modifiers and processing aids, when conventionally utilized in conjunction with one another do not provide either sufficiently high impact strength or desirable processibility, or both, without undue sacrifice of other desirable physical properties. Thus, matching of impact modifiers and processing aids for compatibility and optimum performance is a trial and error task that is usually difficult and time consuming.
For the purpose of developing new materials with desirable properties, miscibility in polymer blends has been laboriously studied. In one approach, it has been proposed to prepare blends of CPVC with other polymers to maximize CPVC's processing characteristics and/or physical properties. While miscible blends have been reported in the literature from time to time, it is well-known that non-miscibility is the rule and miscibility and even partial miscibility is the exception. The prediction of miscibility between polymer pairs is still an art in infancy. Miscibility is believed to be dependent upon a number of factors that include interactions between functional moieties in or pendant from one or more of the polymers, hydrogen bonding, and the like. Various suggestions have appeared for assisting in the selection of miscible polymer pairs including an application of Flory's equation of state as set forth by L. P. McMaster, Macromolecules, 6, P. 760, (1973).
Other suggestions for useful tools in assessing miscible polymer pairs have included normal or three-dimensional solubility parameters, inverse gas chromatography, crystalization characteristics of polymer blends, or evaluation of glass transition temperature shifts, as suggested by L. M. Robeson, Polymer Engineering and Science, 24. p. 589 (June 1984). The statement that the prediction of miscible polymer pairs is an art rather than a science, is demonstrated by, for example, the fact that chlorinated polyethylene having 42 weight percent chlorine is miscible in polyvinyl chloride, while chlorinated polyethylene having a chlorine content less than 42 percent is immiscible in polyvinyl chloride as shown by Robeson, supra at p. 588.
The occurrence of lower critical solution temperature (LCST) behavioral phenomena is commonly associated with the miscibility of polymers. Polymer blends that exhibit LCST-type behavior exist as a miscible blend P (i.e., single phase) below its LCST, and separates into polymer constituents P.sub.1 and P.sub.2 (i.e., two phases) above its LCST.
Miscible blends of polymers have found utility in providing enhanced properties such as plasticization, tensile strength, melt processibility, and enhanced resistance to heat distortion. However, while miscible blends may be utilized to enhance one or perhaps two of the foregoing properties, those skilled in the art have long sought for a dual function modifier having sufficient miscibility to function as a processing aid during processing and at the same time possessing sufficient properties to function as an impact modifier in the finished end product. A desirable dual modifier should impart miscibility for good processibility and have the capability to induce incompatibility for good impact properties in the final product. However, at first glance, it would seem that these properties are mutually exclusive of one another. Accordingly, there is a need for a composition and method therefor to solve these problems.